1. Field of the Invention
This invention relates to fabrication of microelectronic circuit patterns, and more particularly to a method for controlling impingement of an electron beam on a target according to a predetermined pattern of integrated circuitry to be formed.
2. Description of the Prior Art
In fabricating integrated circuits by use of electron beam technology, the beam size and current constitute a significant factor in the efficient production of the microelectronic patterns. In the prior art, the microcircuit patterns are written by an electron beam machine in a vector scanning mode under computer control. The use of a computer to control electron beam microcircuit fabrication is well known in the prior art. For example, a computer controlled electron beam microcircuit fabrication system is described in "A Computer-Controlled Electron-Beam Machine for Microcircuit Fabrication", by T. H. P. Chang and B. A. Wallman, in IEEE Transactions on Electron Devices, May 1972, at pages 629-635.
In the vector scan mode of computer controlled circuit pattern writing, the pattern is divided into a series of rectangles and parallelograms. The electron beam is then controlled by the computer to access these rectangles and parallelograms sequentially and to expose the inside of them. At present, the exposure is performed by scanning the beam of a final size inside each rectangle and polygon in a raster manner as shown in FIG. 1. In order to ensure good edge sharpness and accuracy, a relatively small beam size has to be used, typically one-fourth to one-fifth of the smallest linewidth of the pattern.
Throughput of such a system is dependent on the speed at which the semiconductor wafer can be exposed by the electron beam. Since exposure speed is limited by the current delivered to the wafer surface by the beam and beam current is determined by beam size, one of the major drawbacks of such a system is the limitation on throughput necessitated by the small beam size required to ensure edge sharpness and accuracy. Prior art systems attempted to solve this problem by making trade-offs between beam size on the one hand and edge definition and accuracy on the other. It was not feasible in the prior art to enlarge the size of the electron beam used to fill in the inside of the circuit pattern because the machine had to be powered down to change lens in order to change the beam size. Therefore, the prior art electron beam microcircuit fabrication machines were restricted to a speed determined by the electron beam size required for proper edge definition and accuracy.